The present invention relates to a method of creating a material suitable for use in curtain coating. The invention also relates to a method by which one or more viscous liquid compositions may be coated on to a material such as a continuously moving web of material, as in the manufacture of photographic material such as films, photographic papers, magnetic recording tapes, adhesive tapes, etc.
Curtain coating is a method of coating used extensively in the manufacture of photographic material and products as described in U.S. Pat. Nos. 3,508,947 and 3,632,374. In this method a free-falling liquid curtain of a coating composition is allowed to impinge against a continuously moving web of material. U.S. Pat. Nos. 3,508,947 and 3,632,374 disclose systems in which curtain coating of aqueous gelatin solutions is used.
It is known that wetting in a coating process operates in one of at least two regimes. These are: normal wetting where the coating liquid wets the entire surface of the material being coated, and wetting where a thin film of air is entrained, but is disrupted by the surface topography of the material and subsequently dissolves. U.S. Pat. Nos. 6,099,913 and 6,103,313 are examples of systems in which the synergistic application of a coating composition having a high viscosity and a web to be coated with a specified degree of roughness enable high coating speeds to be achieved, i.e., Roughness Assisted (RA) wetting.
On a rough support, although air-entrainment occurs at approximately the same speed as on a smooth support, provided the coating and material parameters are in the correct range, then coating proceeds until a higher speed, at which point a sudden and gross failure occurs.
It is possible at present to estimate, for a given coating parameter set, that RA wetting is possible, and in addition that if RA wetting is operating, at which higher speed gross failure is expected. Currently available systems only take account of the average peak-to-peak height Rz of the surface of a substrate.
The parameter Rz is a ten point average peak-to-peak roughness measure as shown in FIG. 1 (DIN4768). A sample of the surface of the material is defined and the surface topography determined. The sample is then split up into 5 components of length Im and an average peak-to-peak distance is calculated. Rz is calculated as       1    5    ⁢            ∑              i        =                  1          ⁢                      to            ⁢            5                                      xe2x80x83              ⁢          R      zi      
in which Rzi is the peak-to-peak distance in the ith component.
Whilst it is clear that a constraint on this parameter is a necessary condition further specification is desired Considering a surface composed of beads of a common size, any density of beads up to a limit (defined by the length Im in FIG. 1) will give the same value for Rz, whereas for each density of beads there is a further characterising roughness of the material which is unaccounted for in known systems. In other words, as the beads are spaced further apart, although Rz will remain constant until a spacing determined by Im is reached, the surface topography changes. The propensity for RA wetting will fall, since as the spacing increases the surface will tend to behave more like a smooth surface with isolated perturbations.
Problem to be Solved by the Invention
A method of creating a material is desired on which it is known that RA wetting can occur. A method is required that enables determination of whether or not a particular material is suitable for RA wetting. In particular, a method is required of determining the suitability of a substrate for RA wetting relative to the surface topography.
The method is required for use in the creation of substrates suitable for use in the manufacture of, amongst others, photographic material such as films and photographic paper, magnetic recording tapes, adhesive tapes, inkjet receiver materials etc.
A method is required that enables, for a given set of coating conditions and coating compositions, determination of whether or not the surface to be coated is capable of RA wetting. In addition a method is also required that enables identification of which mechanism of coating, normal wetting or RA wetting, is operating. This enables the appropriate application of a model to predict air-entrainment or gross failure speed in curtain coating.
According to a first aspect of the present invention, there is provided a method of creating a material for use as a material to be coated by curtain coating, comprising the step of creating a surface texture of the material such that when the material is coated with a freely falling curtain formed of a composite layer of one or more coating compositions that impinges at a point of impingement against a continuously moving receiving surface of the material having a roughness Rz (DIN4768) in which the coating composition forming the layer adjacent to the receiving surface has a viscosity, xcex7, measured at a shear rate of 10,000 sxe2x88x921 such that when combined with the roughness Rz, the curtain gives a value of a specifying parameter xcfx86xe2x96xa1 that is greater than 1.
The specifying parameter xcfx860 is defined by             φ      0        =          1818      ⁢              xe2x80x83            ⁢              σ                  1          /          3                    ⁢              R        z            ⁢              {                              1            0.0002                    ⁢          η          ⁢                      xe2x80x83                    ⁢          U          ⁢                      xe2x80x83                    ⁢          cos          ⁢                      xe2x80x83                    ⁢                      θ            ⁡                          (                              1                +                                  Ω                  ⁢                                                            ρ                      ⁢                                              xe2x80x83                                            ⁢                      Q                                                              20                      ⁢                                              xe2x80x83                                            ⁢                      η                                                                                  )                                      }              ;
in which
"sgr" is the surface tension (Nmxe2x88x921) of the layer adjacent to the receiving surface;
xcex7 is the viscosity (Pas) measured at a shear rate of 10000 sxe2x88x921 of the layer adjacent to the receiving surface;
xcex8 is the angle formed between the curtain and a normal to the receiving surface at the point of impingement;
xcfx81 is the average density (Kg/m3) of the one or more coating compositions;
Q is the total volumetric flow rate per unit width (m2/s) of the curtain;
xcexa9 is a Lorentzian factor which reduces the influence of momentum when flow rate is high or viscosity is low, the height of the composite layer at a distance xcex from the point of impingement, in which xcex is the average periodicity of the surface texture, is less than or equal to Rz.
Preferably, the condition                     R        z                    λ        2               greater than                             ρ          ⁢                      xe2x80x83                    ⁢          gH                          4          ⁢                      xe2x80x83                    ⁢          γ                    ⁢              xe2x80x83            ⁢      is      ⁢              xe2x80x83            ⁢      satisfied        ,
wherein:
xcfx81 is the average coating composition density;
xcex3 is the surface tension of the layer adjacent to said receiving surface; and
H is the curtain height.
In one example, the surface texture of the material is intrinsic to a substrate of the material. Alternatively, the texture of the material is created by embossing a surface thereof or the roughness of the material is created by the inclusion of dispersed particulates in a subbing formulation.
Preferably, the step of creating a surface texture of the material is performed such that a parameter xcfx86∈ is defined by
xcfx86∈=xcfx860+1818"sgr"1/3Rz{xc2xd∈∈0E2}
in which
xcfx860 is the specifying parameter defined according to the first aspect of the present invention;
∈0 is the permittivity of free space;
∈ is the dielectric constant of an ambient gas, and
E is the field strength (V/m) of an electrostatic field provided at the surface of the composite layer of one or more coating compositions at the point of impingement, preferably, between 3 and 30 kV/mm.
Preferably, the Lorentzian factor xcexa9 is defined by;   Ω  =            25              25        +                              (                                                            ρ                  ⁢                                      xe2x80x83                                    ⁢                  Q                                η                            -              8                        )                    2                      .  
The electrostatic field may be provided by a voltage of between 200V and 2000V (preferably between 600V and 1500V) between the one or more coating compositions and a backing surface of the receiving surface. In one example, the electrostatic field is generated by charges on the receiving surface.
The receiving surface is preferably a web made of a material selected from the group consisting of paper, plastic films, resin-coated paper, clay-coated paper, calendered paper, synthetic paper, plastic films overcoated with a subbing layer containing surfactant. If the material is a photographic material, the one or more coating compositions may comprise photographic emulsions, protective layers, filter layers or the like.
According to a second aspect of the present invention, there is provided a method of curtain coating a material, comprising the step of forming a freely falling curtain of a composite layer arranged to impinge against a continuously moving receiving surface of the material in which the material is created according to the method of the first aspect of the present invention.
Preferably, the roughness Rz of the material is greater than 2 xcexcm and/or less than 20 xcexcm. More preferably, the roughness Rz is greater than 4 xcexcm and less than 10 xcexcm. It is preferred that the viscosity of the coating material forming the wetting layer has, measured at a shear rate of 10,000 sxe2x88x921, is between 10 mPas and 270 mPas.
In one example of the present invention, an electrostatic field is used to improve traction between the one or more coating compositions and the material being coated. In this case, the coating composition forming the layer adjacent to the receiving surface of the material has a viscosity, xcex7, measured at a shear rate of 10,000 sxe2x88x921 such that when combined with said roughness Rz, the curtain gives a value of a specifying parameter xcfx86∈ that is greater than 1.
The specifying parameter xcfx86∈ is defined by
xcfx86∈=xcfx860+1818"sgr"1/3Rz{xc2xd∈∈0E2}
in which:
xcfx860 is the specifying parameter defined in accordance with the first aspect of the present invention;
∈0 is the permittivity of free space;
∈ is the dielectric constant of an ambient gas; and
E is the field strength (V/m) of an electrostatic field provided at the surface of the composite layer of one or more coating compositions at the point of impingement.
According to a third aspect of the present invention, there is provided a method for curtain coating, comprising the step of forming a composite layer of one or more coating compositions. A freely falling curtain is formed from the composite layer and impinged (at a point of impingement) against a continuously moving receiving surface to be coated having a roughness Rz (DIN4768).
The coating composition forming the layer adjacent to the receiving surface, is provided with a viscosity, xcex7, measured at a shear rate of 10,000 sxe2x88x921 such that when combined with said roughness Rz, the curtain gives a value of a specifying parameter xcfx860 that is greater than 1.
The specifying parameter xcfx860 is defined by             φ      0        =          1818      ⁢              xe2x80x83            ⁢              σ                  1          /          3                    ⁢              R        z            ⁢              {                              1            0.0002                    ⁢          η          ⁢                      xe2x80x83                    ⁢          U          ⁢                      xe2x80x83                    ⁢          cos          ⁢                      xe2x80x83                    ⁢                      θ            ⁡                          (                              1                +                                  Ω                  ⁢                                                            ρ                      ⁢                                              xe2x80x83                                            ⁢                      Q                                                              20                      ⁢                                              xe2x80x83                                            ⁢                      η                                                                                  )                                      }              ;
in which:
"sgr" is the surface tension (Nmxe2x88x921) of the layer adjacent to the receiving surface;
xcex7 is the viscosity (Pas) measured at a shear rate of 10000 sxe2x88x921 of the layer adjacent to the receiving surface;
xcex8 is the angle formed between the curtain and a normal to the receiving surface at the point of impingement;
xcfx81 is the average density (Kg/m3) of the one or more coating compositions;
Q is the total volumetric flow rate per unit width (m2/s) of the curtain;
xcexa9 is a Lorentzian factor which reduces the influence of momentum when flow rate is high or viscosity is low; and
a height function h(x) defines the variation of the height of the composite layer with distance x from the point of impingement, such that h(x)xe2x89xa6Rz at x=xcex in which xcex is the average periodicity of the roughness of the receiving surface
According to a fourth aspect of the present invention, there is provided a method of identifying a material suitable for use in curtain coating using roughness assisted wetting, comprising the step of modelling a height function h(x) of the variation of the height of a composite layer (to be coated onto a material) with distance x from the impingement point of the composite layer on the material, the material being identified as suitable for the coating only if, h(x)xe2x89xa6Rz at x=xcex in which xcex is the average periodicity of the roughness of a receiving surface of the material. Rz (DIN4768) is the roughness of the material.
The height function h(x) is modelled from a freely falling curtain of a composite layer that impinges at an impingement point against a continuously moving receiving surface of a material having a roughness Rz (DIN4768) in which the coating composition forming the layer adjacent to said receiving surface has a viscosity, xcex7, measured at a shear rate of 10,000 sxe2x88x921 such that when combined with said roughness Rz, the curtain gives a value of a specifying parameter xcfx860 that is greater than 1.
The specifying parameter xcfx860 is defined by             φ      0        =          1818      ⁢              xe2x80x83            ⁢              σ                  1          /          3                    ⁢              R        z            ⁢              {                              1            0.0002                    ⁢          η          ⁢                      xe2x80x83                    ⁢          U          ⁢                      xe2x80x83                    ⁢          cos          ⁢                      xe2x80x83                    ⁢                      θ            ⁡                          (                              1                +                                  Ω                  ⁢                                                            ρ                      ⁢                                              xe2x80x83                                            ⁢                      Q                                                              20                      ⁢                                              xe2x80x83                                            ⁢                      η                                                                                  )                                      }              ;
in which:
"sgr" is the surface tension (Nmxe2x88x921) of the liquid layer adjacent to the receiving surface;
xcex7 is the viscosity (Pas) measured at a shear rate of 10000 sxe2x88x921 of the composition adjacent to the receiving surface;
xcex8 is the angle formed between the curtain and a normal to the receiving surface at the point of impingement;
xcfx81 is the average density (Kg/m3) of the one or more coating compositions;
Q is the total volumetric flow rate per unit width (m2/s) of the curtain; and
xcexa9 is a Lorentzian factor which reduces the influence of momentum when flow rate is high or viscosity is low.
According to a further aspect of the present invention, there is provided a method of creating a material for use as a material to be coated by curtain coating, comprising the step of creating a surface texture of the material such that when the material is coated with a freely falling curtain formed of a composite layer of one or more coating compositions that impinges at a point of impingement against a continuously moving receiving surface of the material using roughness assisted wetting, the height of the composite layer at a distance xcex from the point of impingement, in which xcex is the average periodicity of the surface texture, is less than or equal to Rz.
Advantageous Effect of the Invention
The present invention provides a method of creating a material having a surface topography suited for RA wetting during curtain coating. Therefore, if the coating parameters are known the invention enables accurate determination of whether or not any particular material is suitable for RA wetting. The method is accurate since the determination of suitability for RA wetting is in dependence on surface topography, which can be measured accurately.
The method enables materials to be created particularly suitable for use in the manufacture of, amongst others, photographic material such as films and photographic paper, magnetic recording tapes, adhesive tapes, etc. In the manufacture of photographic material, a substrate material may be coated with gelatine based compositions. High coating speed is desirable since this reduces the cost per unit length of material coated and the present invention therefore provides a method that enables the cost of manufacture of photographic material to be controlled.